The role of the serotonergic neuronal system on blood pressure of normotensive WKY rats and spontaneously hypertensive rats (SHR) was studied by injecting L-tryptophan, the metabolic precursor to brain serotonin. It was observed that the dose- and temporal effects of tryptophan on blood pressure were not related to the effects of tryptophan on serotonin levels throughout the brain. The dextro-rotary isomer of tryptophan was no different from L-tryptophan in its ability to increase brain serotonin but D-tryptophan does not alter blood pressure in SHR or WKY. Taken together, these results indicate that the cardiovascular effects of L-tryptophan are not mediated by brain serotonin.